1. Field of the Invention
The present invention relates to a display device and its associated drive control method. More particularly, the present invention relates to a display device and a drive control method for this display device comprising a display panel constituted with an array of plural display pixels containing current control type light emitting devices for performing a light emitting operation at a predetermined luminosity gradation by supplying current corresponding to the display data.
2. Description of the Related Art
Conventionally, a light emitting device type display (display device) comprising a display panel consisting of a two-dimensional array of display pixels equipped with current control type light emitting devices for performing a light emitting operation at a predetermined luminosity gradation corresponding to a current value of the drive current supplied to such as organic electroluminescent devices (hereinafter denoted as “organic EL devices”), Light Emitting Diodes (LEDs), etc. is well-known.
Particularly, a light emitting device type display which applies an active-matrix drive method as compared with a Liquid Crystal Display (LCD) which is widely utilized in various electronic devices including portable devices in recent years provides faster display response speed and there is no viewing angle dependency. In addition, higher luminance, higher contrast, and highly detailed display image quality, etc. are practicable. Since backlight is not needed like in the case of an LCD, this especially predominant feature increases the possibilities for further thin-shaped, light-weight, low-power display devices. Accordingly, the light emitting device type display applying the active-matrix drive method is intensively researched and developed as “the next generation” display.
Besides, in such a light emitting device type display, the drive control mechanism and control method for performing light emitting control of the above-mentioned current control type light emitting devices has been variously proposed. In addition to the above-stated light emitting devices for each of the display pixels which constitute the display panel, an apparatus comprised with a driver circuit (light emitting driver circuit) consisting of a plurality of switching circuits for performing light emitting control of these light emitting devices is well-recognized.
FIG. 23 is an outline configuration diagram showing a substantial part of a light emitting device type display in conventional prior art. FIG. 24 is an equivalent circuit diagram showing an example configuration of a display pixel (light emitting driver circuit and a light emitting device) applicable to a light emitting device type display in conventional prior art.
The active-matrix type organic EL display devices in conventional prior art, as seen in FIG. 23 outline, have a configuration comprising a display panel 110P with a plurality of display pixels EMp arranged in matrix form near each intersecting point of a plurality of scanning lines SL (selection lines) and data lines DL (signal lines) situated in row and column directions; a scan driver 120P (scanning line driver circuit) connected to each of the scanning lines SL; and a data driver 130P (data line driver circuit) connected to each of the data lines DL. A light emitting driver circuit DCp for each display pixel EMp, as shown in FIG. 24, comprises a Thin-Film Transistor Tr111 (TFT; hereinafter denoted as “thin-film transistor) in which the gate terminal is connected to the scanning lines SL, along with the source terminal and drain terminal respectively connected to the data lines DL and a contact N111; a thin-film transistor Tr112 in which the gate terminal is connected to the contact N111 and ground potential Vgnd is applied to the source terminal; and the organic EL devices OEL (current control type light emitting devices) configuration has the anode terminal connected to the drain terminal of the thin-film transistor Tr112 of this light emitting driver circuit DCp and low voltage Vss of lower electric potential than the ground potential Vgnd is applied to the cathode terminal.
Here, in FIG. 24, a storage capacitor Cp is connected and formed between the gate< >source of the thin-film transistor Tr112. Also, the thin-film transistor Tr111 is composed of an n-channel type thin-film transistor and the thin-film transistor Tr112 is composed of a p-channel type thin-film transistor.
In such a configuration of the display device comprising the display panel 110P consisting of the display pixels EMp, initially, by sequentially applying a high-level of the scanning signal Vsel to each row of the scanning lines SL from the scan driver 120P, the thin-film transistor Tr111 of for every row of the display pixels EMp (light emitting driver circuits DCp) perform an “ON” operation and the display pixels EMp are set in a selective state.
Synchronizing with this selection timing, the data driver 130P generates gradation signal voltage Vpix corresponding to the display data and by being applied to the data lines DL of each column, the gradation signal voltage Vpix is applied to contact N111 (namely, the gate terminal of thin-film transistor Tr112) via the thin-film transistor Tr111 of each display pixel EMp (light emitting driver circuits DCp). Accordingly, the thin-film transistor Tr112 performs an “ON” operation by the continuity condition (switch “ON/OFF” operations) corresponding to the gradation signal voltage Vpix. Predetermined light emitting drive current from ground potential Vgnd flows to the low voltage Vss via the thin-film transistor Tr112 and the organic EL device OEL. The organic EL device OEL performs a light emitting operation at a luminosity gradation which corresponds to the display data.
Subsequently, by applying a low-level of the scanning signal Vsel to the scanning lines SL from the scan driver 120P, the thin-film transistor Tr111 of each row for every line of the display pixels EMp performs an “OFF” operation and the display pixels EMp are set in a non-selective state. Thus, the data lines DL and the light emitting driver circuits DCp are electrically isolated. At this stage, based on the voltage which is applied to the gate terminal of thin-film transistor Tr112 and stored in the storage capacitor Cp, the thin-film transistor Tr112 maintains an “ON” state like the above-mentioned selective state. Predetermined light emitting drive current from ground potential Vgnd flows into the organic EL devices OEL via the thin-film transistor Tr112 and a light emitting operation continues. This light emitting operation is controlled to perform a one frame period continuance, for example, until the gradation signal voltage Vpix corresponding to the next display data is applied (written-in) to each row of the display pixels EMp.
Because such a drive control method controls the current value of the light emitting drive current flow to the organic EL devices OEL by adjusting the voltage (gradation signal voltage Vpix) applied to each display pixel EMp (gate terminal of the thin-film transistor Tr112 in the light emitting driver circuits DCp) and performs a light emitting operation at a predetermined luminosity gradation, this technique is known as a voltage assignment method (or voltage application method).
Incidentally, in the display pixels EMp comprising the light emitting driver circuits DCp that utilize such a voltage specification method, the thin-film transistor Tr111 has a selection function and the thin-film transistor Tr112 has a light emitting drive function. When variations and fluctuations (deterioration) are produced in the device characteristics (channel resistance, etc.) depending on the external environment (such as the surrounding temperature), usage time, etc., the light emitting drive current supplied to the light emitting devices (organic EL devices OEL) fluctuates. This presents a problem in that the desired light emitting characteristic (displayed at a predetermined luminosity gradation) is difficult to achieve and maintain stably over a long period of time.
Additionally, when each display pixel is made smaller for adding more pixels to achieve higher resolution of the display panel, variations in the operating characteristics (current between source< >drain), etc.) of the thin-film transistors Tr111 and Tr112 constituted in the light emitting driver circuits DCp become greater. Thus, there is a problem in that proper gradation control cannot be accomplished as variations in the light emitting characteristics of each display pixel are generated which causes deterioration of the display image quality.
Therefore, as a technique for solving such problems, a configuration of the light emitting driver circuits corresponding to a drive control method referred to as a current application method (or current assignment method) is known. Also, although the configuration example of the display pixels (light emitting driver circuits) corresponding to this current application method will be explained in the “DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS”, the following is an outline of the configuration and operation (function).
Accordingly, in the light emitting driver circuits applied to a display device corresponding to the current application method comprise a drive current control circuit (equivalent to the thin-film transistor Tr112 and the storage capacitor Cp mentioned above) which controls the current value of the light emitting drive current supplied to the light emitting devices (for example, the organic EL devices OEL, etc. mentioned above) and its supply state. Specified gradation current of a current value corresponding to the display data is directly supplied from the data driver to that drive current control circuit. Based on the voltage stored according to that current, the current value of the above-mentioned light emitting drive current and its display state are controlled. Thus, this circuit is configured so that a light emitting operation of the light emitting devices is continuously performed at a predetermined luminosity gradation.
As a result, in the light emitting driver circuits that utilize the current application method, the operation constitutes implementing both the function (current/voltage conversion function) which converts the current level of gradation current corresponding to the display data supplied to each display pixel by the drive current control circuit and the function (light emitting drive function) which supplies light emitting drive current that has a predetermined current value based on that voltage level to the light emitting devices. The variation of operating characteristics between plural thin-film transistors as shown in FIG. 24, for example, by forming the drive current control circuit with a single active device (thin-film transistor) has an advantage in that the influence exerted on the light emitting drive current can be controlled.
However, a light emitting driver circuit that employs the current application method described above has a problem as explained below.
Notably, in the light emitting driver circuits of the current assignment method, when (at time of low gradation display) writing the gradation current in each of the display pixels based on display data at the lowest or a relatively low luminosity gradation, it is necessary to supply signal current to each display pixel which has a low current value corresponding to the luminosity gradation of the display data.
Here, the operation which writes display data (gradation current) in each display pixel is equivalent to charging the capacity component (parasitic capacitance; originating in the interconnect capacitance, storage capacitor set in the display pixels, and the like) which is parasitic in the data lines up to predetermined voltage. Because this parasitic capacitance is a capacity component attached to the data lines and equivalent even if positioned anywhere (of the display pixels) above the data lines, when supplying gradation current based on the same luminosity gradation substantially the same write-in time interval is necessary.
Therefore, for example, when the number of scanning lines increases due to enlargement, higher resolution, etc. of the display panel, a selection period (namely, write-in time to each display pixel) of each the scanning lines will be set for a relatively brief interval and also the redesigned wiring length for the data lines becomes longer. When the number of display pixels connected to these data lines is increased, the above-mentioned parasitic capacitance becomes higher to the extent that particularly the current value of the gradation current becomes lower (namely, like the case of a low gradation display) as well as this parasitic capacitance is charged in briefly set write-in time intervals. As a result, write-in deficiencies occur due to the inability to fully write-in the display data to each display pixel.
Accordingly, the current value of the light emitting drive current supplied to the light emitting devices (organic EL devices) of each display pixel becomes lower as compared with the gradation current at the time of write-in (write-in current). Thus, this situation produces an unmanageable problem in executing a light emitting operation at the appropriate luminosity gradation corresponding to the display data which causes deterioration of the display image quality. Also, an in-depth simulation result relating to this problem will be explained later in the “DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS.”